This invention relates generally to pressurized fluid dispensers and more specifically, to pressurized fluid dispensers using an elastic bladder to pressurize the contents.
One application for a bladder-type dispenser is for injecting liquid medications and other fluids into the human body. When injecting fluids into the human body, generally a certain flow rate is prescribed by a physician. Presently a drop counter is used to accomplish this task. Drop counters typically comprise a bottle having a medication in fluid form, which is hung at a height above the patient. The flow rate is adjusted by adjusting a flow control valve until the appropriate number of drops pass out of the valve over a period of time.
The drop counter apparatus for dispensing fluids has several shortcomings. The most prominent shortcoming is inaccuracy in the flow rate of the fluid into the body. The bottle can be hung at various heights in relation to the body and, as the bottle empties, the level of the fluid in the bottle changes. Both of these factors change the total head of the system, which, in turn, changes the flow rate of the system. In addition, different surface tension and viscosity are associated with each fluid. The surface tension and viscosity determine the size of the drops formed by the fluid. The variation in total head and the different sizes of drops can result in inaccuracies of 15 to 20% above or below the prescribed flow rate at any given time.
Drop counters also have other disadvantages in both field and hospital applications. Whenever a patient has to be moved the drop counter must also be moved. Often, an additional person must carry the bottle of fluid or medication during the patient's transport.
Because of the inaccuracies associated with the drop counters, other means have been used in an attempt to inject liquid medications into the body. Pumps increase the accuracy of the flow rate of fluid into the body; however, the motion of the fluid as it is pumped interferes with the systolic motion of the blood stream. In other words, a pump injects fluid at an uneven flow rate.
Pressurizing the fluid has been looked to as a solution. However, prior art attempts at pressurization have met with problems as well. Gas cannot be used with the fluid, since the gas and fluid may react with each other, and it is vitally important that the medication not react with other substances. Furthermore, gas pressurization also runs the risk of inadvertently injecting gas into a person's body, which can be fatal.
Pressurization by use of an expandable bladder has been attempted in the past but has encountered problems. A prominent problem of the previous attempts to use a bladder has been a variation in flow rates as the bladder nears the empty condition. The pressure drops drastically as the bladder empties and, as a result, the flow rate also drops. Several of the prior art devices have attempted to address this problem; however, they have had limited success.
The present inventor is also among the inventors of several of the prior art devices using bladders. The device disclosed in U.S. Pat. No. 3,506,005, and an improvement on that device disclosed in U.S. Pat. No. 3,698,595, both of which were co-invented by the present inventor, use a bladder mounted axially on a mandrel. These devices not only require a specially manufactured bladder but also are difficult to assemble since mounting the bladder on a mandrel requires tedious alignment of the bladder with the mandrel. The valve mechanism necessary to control the flow rate out of the bladder in those devices is also very complex. The valve requires a number of specially manufactured parts and also uses a metal spring to maintain the valve in a closed position. The metal spring is not isolated from the fluid and thus a chemical reaction, such as oxidation, may alter the fluid flowing past the spring. Furthermore, the valve allows variation in the flow rates as the pressure drops.
The device disclosed in U.S. Pat. No. 3,907,169, of which the present inventor was also a co-inventor, uses a bladder mounted on the outside surface of a pivoting curved spatulate member. This device eliminates the difficulties of aligning the bladder upon a mandrel during manufacture; however, the pivoting of the spatulate member causes other problems. Since the spatulate member can move as the bladder is filled or emptied, the bladder must be attached to the spatulate member. The bladder has to be specially formed to accommodate specially formed clips for attaching the bladder to the spatulate member. The devices uses a complex valve similar to that disclosed in the previous patents. The problems associated with the valve in the prior patents are also associated with the device described in this patent.
Thus, there has been a need for a bladder-type dispenser that can pressurize a fluid and maintain a substantially constant flow rate as the bladder empties, is easily manufactured and assembled, and is easy to use. In addition, there is a need for a valve system on a bladder-type dispenser having simple, reliable operation and having no metal components in contact with the fluid inside the dispenser.